Selective internal radiation therapy (SIRT) is a therapeutic option for primary and metastatic liver tumors. Microspheres containing Yttrium 90, a beta-emitting radionuclide, are administered into the hepatic artery allowing selective internal radiation of a liver tumor. SIRT-related complications may appear due to migration of the radiation microspheres to organs distant from the tumor site. In order to prevent these complications, unintended non target embolization of Yttrium microspheres has to be avoided. However, data from external-beam radiation therapy (EBRT) suggests that the stomach/small bowel may actually be less radiosensitive than the liver. Gastric ulcers, a well-known SIRT-related complication, may therefore not only be caused by local radiation but also by unusual accumulation of microspheres in the submucosa and small vessel damage. We herein report a more than two- year-long persisting, highly symptomatic, non-neoplastic ulceration of the gastric antrum leading to pyloric stenosis caused by SIRT therapy with Yttrium 90 microspheres for the treatment of intrahepatic cholangiocellular carcinoma. The chronic courses of the ulcer disease together with the specific histological features highlight the pivotal role of radiation-induced small vessel damage in SIRT-induced adverse events.